The invention relates to a deflection mirror. Furthermore, the invention relates to a projection exposure apparatus for use in microlithography comprising such a deflection mirror.
Projection exposure apparatuses for microlithography for the EUV wavelength range rely on the mirrors used having a high reflectivity since, firstly, the product of the reflectivity values of the individual mirrors determines the total transmission of the projection exposure apparatus and since, secondly, EUV light sources are limited in terms of their light power.
Mirrors for the EUV wavelength range having high reflectivity values are known from DE 101 55 711 A1, for example. The mirrors described therein consist of a layer system applied on a substrate and having a sequence of layers. Such mirrors having a layer system (so-called multilayer) are generally operated with virtually normal incidence, i.e. at angles of incidence of between 0° and 45°. With virtually grazing incidence, i.e. at angles of incidence of between 45° and 90°, by contrast, mirrors having a single layer are generally used, said single layer consisting of a metal.
In order to increase the reflectivity for angles of incidence in the transition range between normal incidence and grazing incidence in the case of mirrors having predominantly grazing incidence, therefore, mirrors are known which combine a multilayer with a single layer. In this case, the mirrors are divided into discrete zones. In each of said zones, one of the coatings mentioned can be used. Such mirrors are described as collector mirrors for an EUV light source, for example, in US2010/0284511. The disadvantage of these collector mirrors, however, is that each zone is coated with a dedicated layer system having a homogeneous thickness. This leads to a rising and subsequent falling of the reflectivity in each zone. At the connection locations of the zones, local minima arise in the reflectivity curve, which, although they are insignificant for the use of such a mirror as a collector mirror, nevertheless for the use of such a mirror as a deflection mirror at the end of an illumination optical unit directly upstream of the mask of a projection exposure apparatus they lead to a non-tolerable intensity fluctuation on the mask. Furthermore, a coating in zones is complicated to carry out since in each case only one zone is to be coated. The other zones in this case have to be protected e.g. by covers.